In my prior U.S. Pat. No. 4,205,276, which was obtained jointly with John W. Wright and assigned to the assignee of the present invention, an amplifier is disclosed having a 12 db per octave roll off. The teaching in that patent is incorporated herein by reference. The object of the steep roll off is to obtain substantial gain at the higher audio frequencies, sufficient for Hi Fi performance, and yet produce substantially zero output in the AM radio broadcast band. The circuit disclosed employed a two-pole filter concept with each filter introducing a 6 db roll off. It was found that such a steep roll off resulted in instabilities and a frequency selective feedback loop was incorporated for stabilization.
By employing a very steep roll off the amplifier output signals in the AM radio broadcast band are substantially eliminated. This means that in a very small radio, such as would be available in an integrated circuit (IC) version, the Hi Fi audio amplifier will not introduce spurious output signals that the radio can respond to in the form of noise or interference.